Application Ser. No. 198,768 now U.S. Pat. No. 3,907,971 describes a process for the removal of hydrogen fluoride from a gas stream containing same to the extent that the gases released into the atmosphere can have a significantly lower HF concentration than results from prior systems for the removal of HF from gas streams. The gas stream may be generated as an exhaust gas in the electrolytic production of aluminum.
The improvement described in application Ser. No. 198,768 is based upon the surprising discovery that the efficiency of removal of HF from a gas stream containing same can be substantially increased to a qualitative extent when the contact between the gas phase and the solid phase is carried out in an expanded fluidized bed, i.e., a fluidized bed in which a substantially uniform gradient of solids concentration or density (number of particles per unit volume or solids weight per unit volume) is maintained between the floor of the fluidized bed and the gas outlet therefrom, preferably throughout the height of the chamber, such that no sharp demarcation exists between the fluidized bed layer and any free space above this bed. The gas flow is such that the major proportion of the solids removed from the expanded fluidized bed is entrained by the gases and is separated therefrom, according to this application, by centrifugal action and sedimentation. The adsorbent particles are constituted of alumina which picks up the hydrogen fluoride and recycles it to the bed and to the furnace. The system described in the earliest application mentioned above uses primarily cyclone separation of the particulate matter entrained from the bed by the gases which can be released into the atmosphere substantially free from hydrogen fluoride.
In the subsequent application Ser. No. 488,930, there is described an improvement in which the particles are removed from the gas stream effluent from the expanded bed directly by electrostatic precipitation, it being indeed surprising that the preliminary mechanical separation by cyclones or the like can be eliminated. Surprisingly, the electrostatic precipitator is not overloaded by this action.
In general, the earlier systems deal with processes for the separation of hydrogen fluoride from exhaust gases formed in the production of aluminum by electrolytic techniques, i.e., the exhaust gases from an electrolytic production of aluminum by the dry adsorption of the HF upon alumina in a fluidized-bed reactor into which the gas is introduced as the fluidizing medium in such velocity as to produce a highly expanded fluidized bed as thus defined. That is, the solids concentration decreases from the bottom to the top uniformly over the entire height of the chamber as measured from the bottom to the outlet at the top of the chamber. The major part of the solids is discharged upwardly in entrainment with the gases and the solids are separated in an electrostatic precipitator directly following the fluidized bed in the improved version previously described. By "directly" following the fluidized bed it is meant that there is no intermediate separation of particles from the gas between the outlet of the expanded fluidized bed and the electrostatic precipitator.
However, the system described above has been found to have some problems, particularly since the gases effluent from the electrolytic-aluminum furnace, contain deleterious substances which will be described in greater detail below but which are taken up by the adsorbent and are recycled to the furnace with the latter so that the concentration of these deleterious substances tends to increase in the electrolytic furnace. Of course, where the alumina is not first used as an adsorbent but is charged directly into the furnace, the problem does not arise. Hence, while the systems described above were successful in purifying the air released into the atmosphere, they caused an additional difficulty with respect to return of deleterious substances to the electrolytic aluminum bath. The present system is directed to the avoidance of this problem.
In more general terms, it may be noted that various processes for recovering contaminants from a gas stream have been provided heretofore, whether because it is economical to remove valuable substances or because environmental pollution may occur if these substances are permitted to remain in the gas stream.
The techniques commonly used may be wet processes or dry processes, the wet processes including scrubbing of the gas stream with a liquid and the dry processes including adsorption in fixed or mobile beds and various particle separation techniques such as filtration, sedimentation or cyclonic separation and the use of electrostatic precipitators. As noted in the application Ser. No. 488,930, however, it has hitherto been the belief that electrostatic precipitators should not be used except where solids concentrations are very small indeed and hence only after a preliminary separation of particulates has been effected by one or more of the other techniques mentioned.
A special problem arises when the gas stream contains hydrogen fluoride (HF) not only because of the highly corrosive character of this constituent but also because the fluoride is most valuable in the production of aluminum. HF-containing gases are evolved from furnaces for the electrolytic production of aluminum because complex fluorides, such as cryolite, are used as fluxes for the alumina which constitutes the raw material from which the aluminum is produced. The exhaust gases contain up to about 1000 mg HF/m.sup.3 (STP), depending upon the method by which the gases are drawn off and the degree to which they are diluted, e.g. by air. As a rule, the gases contain less than 100 mg HF/m.sup.3 (STP).
Apart from the techniques described above which have proved to be highly successful in the removal of HF from such gases so as to enable the exhaust gases to be released in a completed nontoxic manner into the atmosphere, various other dry systems have been proposed and are characterized by various drawbacks which should be reviewed in order to place the present improvement in a proper perspective.
For example, in one process dry adsorbents such as limestone, calcium carbonate, slaked lime, quick lime, alumina, activated alumina and magnesia, are introduced into a conduit traversed by the gas stream and leading to a bag filter in which the particles accumulate in a layer which is traversed by the gas. The particles thus remain in contact with the gas for a brief period in the conduit and then are contacted after they form a layer upon the filter surface. The adsorbent is fine-grained and has a mesh size generally smaller than 200 mesh. One difficulty with this system is that segregation of the particles tends to occur and the layer on the vertical pockets of the filter tends to have streaks and regions of different effective thickness so that a homogeneous contact of the gases with the dust cannot be ensured. The contact time between the gas and the particles also tends to be relatively short so that an optimum mass transfer of the HF is not always ensured.
In another process hydrogen fluoride is removed from exhaust gases which contain less than 880 mg HF/m.sup.3 (STP) by passing the gas at a temperature below 200.degree. C through a layer of alumina and periodically removing this layer. The fluoride-enriched alumina is introduced into the electrolytic cells. The layer may be formed upon a bag filter and has the disadvantages of the earlier filtration techniques. In both cases as in the systems described in the aforementioned copending applications, any deleterious substances in the gas are either released into the atmosphere or are recycled to the bath with alumina.
In still another system activated alumina is used to treat the gas in counterflow and this system requires large flow cross-sections and is thus economically unviable since it requires the production of activated alumina first.
Other systems are provided and may use filter bags, dense beds (as contrasted with expanded beds) and the like, none of which has proved to be entirely satisfactory for the reasons already stated or on economical grounds.
The impurities mentioned above which have proved to be substantial problems when recycled to a bath for the electrolytic production of aluminum include finely divided carbon, iron, vanadium, phosphorus, titanium and other metals and their compounds which tend to remain as impurities in the aluminum metal when they are present in significant quantities in the bath. In practice, such impurities are found to be recovered with the alumina used to absorb the HF and are returned to the bath therewith.